In the design of transmitters for ultra-deep submicron CMOS wireless transceivers, it is desirable to minimize transmitter (TX) emissions in one or more receive frequency bands (i.e., RX band noise) while preserving overall transmitter linearity. In a typical design, a transmitter circuit may employ multiple successive gain stages, wherein more gain is assigned to the earlier stages (e.g., close to baseband) than to the later stages (e.g., close to the transmitter output). To reduce RX band noise, an off-chip attenuator (e.g., an output pad) may be used to attenuate the noise during the later gain stages, e.g., prior to amplification by an off-chip power amplifier.
The use of an off-chip attenuator to reduce RX band noise suffers from at least two drawbacks. First, an off-chip attenuator such as an output pad typically provides only a fixed attenuation level, without the option of scalable attenuation. This may limit the flexibility of the off-chip attenuator, as well as increase the number of external components required. Second, since the off-chip attenuator will generally attenuate both TX signal and noise power, the TX signal swing at the input and output of the transmitter gain stage prior to the off-chip attenuator (e.g., a driver amplifier) must be made larger to compensate for the attenuation. This may adversely affect the transmitter linearity, as well as undesirably increase transmitter power consumption.
It would be desirable to provide flexible, low-power techniques for minimizing RX band noise while preserving overall transmitter linearity.